This invention relates to optical fiber cables having improved optical transmission characteristics. More particularly, it relates to lightwave transmission cables with optical fiber ribbon assemblies designed to reduce bending losses.
High capacity lightwave transmission cables frequently comprise multiple optical fibers organized in a ribbon configuration. Optical fiber ribbons are made by arranging two or more optical fibers side-by-side and coating the optical fibers to bind them together in a single planar array. One or more optical fiber ribbons may then be cabled in a single cable for high capacity optical transmission systems. Where more than one ribbon is used, an efficient arrangement is to stack the optical fiber ribbons, and apply a cable jacket to surround and protect the stack. The stack typically has a rectangular cross section. An advantage of stacked optical fiber ribbon cables is that the individual optical fibers remain organized throughout the cable length during the cabling operation, and in use. Thus for relatively small optical fiber count cables the input and output ends of a given optical fiber are easily matched. (For large count cables color-coded coatings on the ribbons or the fibers typically identify the fiber ends.) Another important advantage of stacked ribbon cables is space efficiency. The cable volume required per optical fiber in a stacked ribbon configuration is typically less than that for a given fiber in a loose fiber bundle.
It has long been recognized that bending of optical fibers is a principal signal loss mechanism. The smaller the bend radius (microbend) the more light escapes from the core of the fiber and is lost. When multiple fibers are arrayed in a cable, the microbending problem is influenced by the nature of the array, since bundles of fibers mechanically interact with one another, as well as with the cable sleeve. The use of optical fibers arrayed in ribbons controls that interaction to some degree, but optical fiber ribbons have their own unique microbending behavior. In an optical fiber ribbon with a rectangular cross section, the out-of-plane bending stiffness is significantly lower than the in-plane bending stiffness, giving rise to the so-called preferred bending axis. Among other consequences, this preferential bending characteristic can cause nonrandom stresses on certain fibers in the ribbon during cable loading. These stresses may degrade the signal transmission characteristics of the optical fibers in the cable. Thus optical fiber ribbons present special considerations in cabling.
It is also universally recognized in optical fiber cable design that a preferred approach to controlling microbending losses is to mechanically decouple the optical fibers from the surrounding cable. In this way mechanical impacts and stresses on the cable are not translated, or minimally translated, to the optical fibers. Various techniques have been used to achieve this. Early approaches involved placing the optical fiber(s) loosely in a relatively rigid tube. The object was to allow the fibers to xe2x80x9cfloatxe2x80x9d in the tube. In alternative designs, the optical fibers are coated with a primary coating, typically a polymer coating, and a cable sheath applied over the coating, also typically a polymer. The primary coating in this case is made soft, so that stresses experienced by the cable are inefficiently translated to the optical fibers within the cable. In yet another design aimed at the same goal, the optical fibers are coated with a gel to reduce mechanical coupling between the optical fibers and the surrounding cable sheath. See U.S. Pat. No. 6,035,087, issued Mar. 7, 2000.
The term xe2x80x9cencasementxe2x80x9d as used herein is defined as the primary medium that surrounds the optical fiber ribbon stack.
These techniques as applied to ribbon cable have only moderate success. This is partly due to the tendency of ribbons within the cable to buckle or wrinkle when the cable is moderately bent. The wrinkles form on the inside radius of the bend. Whereas the bend itself may have a relatively large radius, a radius that is above the range where serious microbending losses would occur, the bends of the wrinkles are much smaller, and easily translate to the optical fibers causing microbending loss. Thus a technique for eliminating or minimizing these ribbon wrinkles in optical fiber cables would represent an important advance in the technology.
A particularly thorough discussion of coatings or encasements for optical fiber ribbon cables appears in U.S. Pat. No. 6,317,542 issued Nov. 13, 2001. This patent describes a variety of embodiments wherein conformal encasements are used for optical fiber ribbon stacks. The discussion of optical fiber ribbon stacks is especially relevant to the discussion below, and this patent is incorporated by reference herein. The term stack as used herein includes one or more optical fiber ribbons.
We have discovered that, contrary to conventional practice, increasing the coupling between the optical fibers and the surrounding cable provides unexpected benefits, and reduces the tendency of optical fiber cables to buckle and wrinkle. This effect is especially pronounced if the optical fibers collectively exhibit a preferred bending axis, for example, an optical fiber ribbon. Increased coupling and reduced microbending loss is achieved by a combination of three features. First, a relatively high modulus encasement is used. Second, adhesion between the optical fiber ribbon and the encasement is promoted. The combination of a relatively stiff medium surrounding the optical fiber ribbon and relatively high adhesion between the optical fiber ribbon and the surrounding medium is important to allow stresses on the cable exterior to be translated to the optical fiber ribbon. Translating the stresses to the optical fiber ribbon allows the glass fibers in the optical fiber ribbon to be used as compression strength members. Inhibiting compressive strain on the optical fiber ribbon cable reduces markedly the tendency of the optical fiber ribbon(s) to form wrinkles on the interior of the bend radius. A measure of the effectiveness of this is the shrinkage factor of the encasement with respect to the optical fiber ribbon stack, as will be described in more detail below.